1. Field of the Invention
The present invention relates to the field of flow control valves, and more particularly to a divider-combiner valve.
2. Description of Related Art
When two cylinders are used together to lift a common load, the rod end ports are teed together and the blind end ports are teed together. As oil is directed into the blind end, the cylinders will extend. By reversing the flow, the cylinders will retract. If the loads that each cylinder lifts are equal, the cylinders will move together and stay synchronized.
However, most of the time, these loads can""t be expected to be equal. Whenever the loads are unbalanced, the flow will take the path of least resistance and go to the light loaded cylinder. It will move faster, or in some cases, take all the oil. Where synchronization is desired, this is not acceptable, and additional valving is needed to assure that the cylinders move together. The tees at the rod end ports and blind end ports referred to above are replaced with valves.
Simple divider valves are designed to assure that a single input flow of oil is divided into two equal outputs, regardless of the pressure that either outlet flow is working at. Following is an explanation of how this is done. Whenever flow passes through an orifice, a pressure drop occurs. This drop is predictable and repeatable. If two orificies are of equal size, and an equal amount of flow passes through each, the pressure drop across each orifice will be the same. An increase in flow will increase the drop and a decrease in flow will decrease the drop. This is the principal that divider valves use.
A simple divider valve consists of a spool in a cast iron body. Flow comes into the inlet port to the center of the spool. Orifices allow flow to go to the right and left. If both orifices are of equal size and the loads are equal, half the oil flow will go each way. Flow continues through the center of the spool and out metering holes that connect to each of the outlet ports and on to the two respective cylinders. The spool remains in a centered position.
However, whenever the work loads are not equal as mentioned before, the flow wants to go to the lightest load. As this happens, the increase in flow will increase the pressure drop through this orifice. Because more flow is going towards the lighter load, less oil is going through the other orifice to the heavier loaded cylinder. Again, a change will occur from the desired pressure drop through the respective orifice, on this side it will have less pressure drop.
By analyzing the forces acting upon the spool, it can be seen how the valve accomplishes its metering. All oil entering the center of the spool comes in at a pressure great enough to lift the loads, plus the pressure drop of the flow through the orifice. For an example, the input flow of 16 gpm may come in at 1600 psi. Half the oil goes to the right and left. Orifices are sized to cause a 75 psi drop. 1525 psi would be used to lift the loads. If both loads were equal, this would remain true throughout the complete travel of the cylinders.
When the loads are not equal the following takes place. Continuing with the above example, but reducing the load on one side to only need 500 psi, as flow increases toward the light load, the delta P through the orifice will increase. The pressure at the end of the spool is the inlet pressure less the pressure drop through the orifice. A very small change in flow, for example 0.2 gpm, going to the light side would increase flow to 8.2 gpm and the pressure drop would increase to 80 psi. At the same time, the flow on the heavy side would be reduced to 7.8 gpm and the drop across its orifice would go down to 70 psi. The resulting pressure on the ends of the spool would be 1630 psi on the heavy loaded end and 1620 psi on the light loaded end. This unbalance in pressure will act on the full area of the end of the spool and cause the spool to move toward the light loaded side. As the spool moves, radial metering holes are closed off as they pass over the lands of the casting. This metering, or restricting, adds a second, internal pressure drop to the flow going to the light load. The spool will continue to move towards the light side, adding more and more restriction, until this internal restriction plus the external load equals the external load on the heavy loaded side. Since the total loads, on each side, are now equal, there is no path of least resistance, and flow at each outlet port will be equal.
The spool is now back in equilibrium and will stay there until there is another change in the external loads. The time it takes the valve to react to these changes is almost instant, in the range of milliseconds. However, it must be remembered that the valve spool could be in constant motion if the loads are continually changing.
The accuracy of the divider valve relies upon the forces to reposition the spool; it is desired to maximize these forces. The force is dependent upon the pressure drop across the orifices (or more accurately, the difference in delta P of the two orifices) acting upon the area at the end of the spool. Large differences in delta P are only accomplished by large changes in the outlet flows, which are not desired. The area is dependent on the diameter of the spool. The valve should have a spool with as large a diameter as the design will allow. Comparing two valves (at the same flow and orifice sizes), the one with the larger spool diameter will be the more accurate.
This type of valve only divides the flow in one direction, from inlet to two equal outlet flows regardless of the pressure on the outlet ports. It is necessary to include check valves in the divider if flow is to be returned, as in a cylinder application, from the two xe2x80x9coutletsxe2x80x9d back to the xe2x80x9cinletxe2x80x9d port. This flow is not synchronized.
A combiner valve is available to work in the opposite direction. Two streams of oil are combined equally into one. These type of valves work on the same principal as the divider by sensing the pressure drop across two orifices and using the difference that occurs when the flow is unequal to reposition the spool. Again, check valves are needed for reverse flow.
A pair of divider valves (with checks), or a pair of combiner valves (with checks), is necessary when synchronization in both directions is wanted. For situations where the loads could xe2x80x9crun awayxe2x80x9d, the combiners are required. For double acting cylinders, valves must be used in pairs, two dividers or two combiners, not one of each.
For single acting cylinders, where only one port on each cylinder is available, the set up becomes much more difficult using dividers and combiners. One of each type of valve would be used for each direction of travel. It would be necessary to externally check the flow around the xe2x80x9cunusedxe2x80x9d valve for each direction of flow. A cumbersome circuit, that is impractical, would be required.
A much simpler solution would be a divider/combiner valve. A single valve would synchronize travel in both directions. This type valve would work on either single or double acting cylinders. By placing the valve to combine on the xe2x80x9crun awayxe2x80x9d direction, all applications could be handled.
Divider/combiners have been made for a number of years with certain disadvantages inherent in their design. By looking at the spools of a divider and combiner, it can be seen that the metering is done on the radial holes. But these holes are located differently on each of the different type spools. The metering holes of the divider spool are located towards the outer ends of the spool. The metering holes of the combiner are located near the center of the spool. For a divider/combiner, with only one spool, the divider holes must be the only holes open in the dividing mode. Likewise, these dividing holes must not be open when in the combining mode, but the combiner holes must be.
Until now, there have been two approaches to these possibilities. One design has a metering spool that has two xe2x80x9cshuttlexe2x80x9d spools inside it. When flow comes in the inlet, the xe2x80x9cshuttlesxe2x80x9d are moved towards the ends of the metering spool. The xe2x80x9cshuttlesxe2x80x9d now cover the combiner holes and open the divider holes. The xe2x80x9cshuttlesxe2x80x9d remain in this position as long as the flow is dividing and the whole spool assembly moves as a divider spool. When the cylinders are reversed, the xe2x80x9cshuttlesxe2x80x9d move toward the center and uncover the combiner holes and cover the divider holes. Manufacturers have found it is necessary to use springs to position the xe2x80x9cshuttlexe2x80x9d spools toward the center. They have also used springs on the ends of the whole spool assembly to assure that the spool does not get xe2x80x9clocked upxe2x80x9d at one end or the other of travel. Additional pressure drop through the orifices are required to overcome these spring forces. These pressure increases are inefficient and not necessary except to assure the spools move and do not hang up.
The other design shown, and described in U.S. Pat. No. 4,531,536, has two identical spools. These spools are hooked at the center. This hooking arrangement allows the spools to extend so that the radial holes are in the divider location when flow is in the inlet port. When the oil flow is reversed, the spools move towards each other so that their radial holes are in the combiner location. Again, springs are used to assure the spools do not hang up.
In both of these designs, there is also the potential problem that the spools do not completely shift to their full position. If this occurs, then the xe2x80x9ccorrectxe2x80x9d holes are not covered or open in the first design, or not aligned in the second design. Contamination is the most likely cause of this problem. Extra clearance is often used to assure that the spools move freely within one another. The greater clearance, plus the fact that there is the added clearance between the spools, allows for greater internal leakage. This internal leakage makes the valve less accurate. Also, when the hydraulic flow is stopped, there is a greater transfer of fluid from one cylinder to the other allowing one cylinder to extend and the other to retract.
As a consequence of the foregoing situation, there has existed a long-standing need for a new and improved divider-combiner valve, and the provision of such a construction is a stated objective of the present invention.
Briefly stated, the present invention provides a divider combiner valve using only one spool without any springs. The correct holes open and close by check valves. These are simple and reliable. There is no chance of the holes not being in the correct location. Since there is only one spool, contamination cannot keep the spools from shifting into their correct relationship with each other. The extra pressure drops, needed to overcome the spring forces in the earlier designs, have been eliminated all together. Also, the number of spools has been reduced from two or three down to a single spool, thus reducing the cost to manufacture the finished valve. Because there is only one spool, clearances are closer. There is less internal leakage, more accuracy while the cylinders are in motion. This closer clearance tends to lock the loads rather than transfer oil from one cylinder to the other.